This invention relates to a variable gain tilt control system for use with an optical amplifier, and more particularly with a rare earth doped optical fiber amplifier.
Optical amplifiers and particularly erbium doped optical fiber amplifiers are nearly ubiquitous in optical transmission systems, particularly in the field of telecommunications. Erbium doped fiber amplifiers (EDFAs) have high polarization insensitive gain, low cross talk between signals of different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics allow hundreds of these amplifiers to be cascaded to cover spans of thousands of kilometers of optical fibre. EDFAs as opposed to electronic repeaters are also transparent to data rate, signal format, and wavelength over a limited range, making them useful for wavelength multiplexed (WDM) communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal.
Notwithstanding these generally excellent characteristics, a disadvantage associated with EDFAs is their narrow spectral width and uneven gain band. The useful telecommunications window of an EDFA is approximately 20-30 nm wide, while an ideal amplifier would have a flat spectral gain across the full spectrum which extends from approximately 1520 nm to 1570 nm. The peak wavelength of the erbium gain spectrum varies from about 1530 nm to about 1535 nm depending upon the host glass material. FIG. 1 shows the characteristic gain spectrum of a particular conventional EDFA where it can be seen that the gain as a function of wavelength varies; this variation will be referred to hereinafter as gain ripple. Numerous techniques have been published for widening and flattening the gain spectrum (i.e. reducing the ripple) and include for example co-doping an erbium-doped silica glass fibre with Al2O3; changing the host glass material itself; using various forms of attenuating filters to reduce the gain at the emission peak; and, constructing hybrid devices having two or more different types of serially connected erbium doped fibre and actively adjusting pump conditions independently in each fibre section to compensate for the different gain slopes of each fibre.
In addition to the aforementioned problems and solutions associated with minimizing gain ripple, another significant problem exists to which there have been no simple, inexpensive, and practicable solutions. This other significant problem solved by this invention relates to improving dynamic gain tilt. The term dynamic gain tilt as used hereafter means the variation in gain at one wavelength as a result of changing the gain at any other wavelength via a change in input EDFA operating conditions. Although the techniques described above for minimizing gain ripple can provide a relatively flat spectrum in a specified wavelength band for a specific set of input optical powers and wavelength, the gain equalization performance degrades rapidly when the gain is changed (change in average population inversion levels) from the nominal conditions by changing the input power to the amplifier. One reported solution to this problem is allegedly achieved by a hybrid fibre device having cascaded amplifying stages with different gain spectra and an equal number of pump sources to allow the gain spectra of the individual stages to be effectively tuned independently so that when the total gain is changed, the relative contribution of each stage can be adjusted to arrive at the desired gain, with a resulting gain spectrum having a minimal amount of spectral distortion over the selected wavelength band. As an example, an erbium doped fibre having a positive gain slope may be combined with a different erbium doped fibre having a negative gain slope such that the hybrid device has a nearly flat gain at specific input power conditions. However, if the overall gain of the hybrid device must be changed, the gain slope of each of the constituent states will generally change at different rates when the pump power input to one of the stages is changed. In order to achieve good compensation at the new operating point, the relative gain of each of the constituent gain stages must be readjusted to make the gain slopes compensate each other. In implementing this type of amplifier, one skilled in the art would likely cascade two or more different erbium doped fibre compositions and provide a separate pump source for each amplifying stage at an end of each stage so as to minimize the number of splices and make it as convenient as possible to independently control the pump power to each stage. However, this technique for reducing or improving dynamic gain tilt requires a complex control scheme during operation in which the total power of multiple pump sources must be coordinated in order to realize gain slope compensation over a range of different gains (i.e. to change input power while maintaining a fixed target output power).
U.S. Pat. No. 5,764,406 in the name of Newhouse et at. entitled Hybrid Optical Amplifier Dynamic Gain Tilt incorporated herein by reference, describes a system wherein an erbium doped fibre amplifier device has a dynamic gain tilt that is less then the gain tilt of any of the constituent fibres. The hybrid device has at most one less pumping source than the number of constituent waveguides of the device. The hybrid device automatically provides a change in the pump distribution among the constituent doped waveguide sections so as to achieve a readjustment of the relative gains of the constituent sections. In one embodiment, this invention provides constituent EDFs of different co-dopant compositions that provides an automatic change in the pump distribution or partitioning among the constituent EDF sections so as to achieve a readjustment of the relative gains of the constituent EDF sections.
Although the ""406 patent appears to achieve its intended function, it is a relatively costly and is a complicated solution to dynamically controlling gain tilt.
Most known solutions for correcting for dynamic gain tilt have an associated power loss (approximately 5 dB) and furthermore, increased power is required for extra pumping. Another deleterious result of these systems is an increased resulting noise.
In the L-band, gain flatness is known to be difficult to achieve for operation of an L-band optical amplifier when temperature and power varies. In a paper entitled xe2x80x9cThe gain equalizing method of 1580 nm-band Erbium doped fiber amplifier for wide temperature and wide input power rangesxe2x80x9d Suzuki et al of the Oki Electric Industry Co., Ltd. discloses using automatic gain control (AGC) operation by adjusting the backward pumping power and controlling the input power by a variable attenuator. In a paper entitled xe2x80x9cTemperature dependent behavior of C-band and L-band EDFAs: A comparisonxe2x80x9d F. A. Flood discloses that the temperature dependence of signal emission and absorption cross-sections have greater impact on the L-band output power. Furthermore, it is shown that the L-band temperature coefficient (dB/xc2x0 C.) is comparatively less sensitive to pump wavelength. Thus, it is known that fluctuations in temperature of an erbium doped optical fibre amplifier degrade performance of the amplifier.
It has been discovered, that the tilt of an L-band optical amplifier can be controlled and varied by controlling and/or varying the temperature of the erbium-doped optical fibre within the amplifier. Heretofore, the convention has been to temperature compensate optical amplifiers to ambient so that variation in their operating temperatures did not occur, as a change in operating temperature was viewed as deleterious to the functioning of the device.
Now, in accordance with the discovery that tilt can be controlled and/or controllably varied to achieve a desired tilt, within predetermined limits, amplifiers can be pre-tested to characterize their tilt at and variation in their tilt for various applied temperatures.
Furthermore, dynamic tilt control can be provided by way of heating or cooling the EDF in an L-band optical amplifier by providing a feedback signal corresponding to the tilt of the output signal. In response to the feedback signal temperature of the erbium fibre can be varied dynamically to stabilize or vary the tilt as required.
It is an object of this invention to provide a novel controller and control method for use within rare-earth doped optical amplifiers that can be used to complement a filter that can be placed in-line within an optical amplifier for varying the gain tilt dynamically as the power of the input signal is varied, or that can be used without such a filter in an EDFA system for controlling gain-tilt or curvature of the output response of the amplifier.
It is another object of the invention is to provide an inexpensive controller and controlling method for use in controlling tilt gain of an optical amplifier.
In accordance with the invention there is provided, a plurality of optical amplifiers, each having a substantially same tilt in the presence of a same input optical signal, each of the plurality of optical amplifiers comprising a length of rare-earth doped optical fibre, and a temperature set point at which the rare-earth doped optical fibre is maintained, wherein each one of the plurality of optical amplifiers has a substantially different temperature set point selected for compensating a loss variation of components of each one of the plurality of optical amplifiers to maintain the substantially same tilt of the plurality of optical amplifiers.
In accordance with another aspect of the invention there is provided an optical amplifier kit comprising an optical amplifier for providing a predetermined tilt corresponding to a predetermined constant temperature having: a length of rare-earth doped optical fibre, a pump laser optically coupled thereto, at least one of a heater and a cooler coupled to the length of rare-earth doped optical fibre for maintaining the optical fibre at the predetermined constant temperature corresponding to the predetermined tilt of the optical amplifier, and, a temperature set point at which the amplifier is to be constantly maintained to obtain an output tilt corresponding to the predetermined tilt.
Further, in accordance with yet another aspect of the invention, there is provided a method of setting an optical amplifier for operation at a predetermined tilt or curvature comprising the steps of providing an L-band optical amplifier having a length of rare-earth doped optical fibre, determining a temperature set point of the optical amplifier for providing the predetermined tilt or curvature at a respective temperature, and constantly maintaining the optical amplifier at said temperature set point.
In accordance with the invention there is further provided, a method of operating an optical amplifier at a predetermined tilt or curvature comprising the steps of providing an optical amplifier having a length of rare-earth doped optical fibre, determining a temperature set point that corresponds to the predetermined tilt or curvature for compensating a loss variation of components of the optical amplifier, and constantly operating the optical amplifier at said temperature set point.
In accordance with the invention, a method of characterizing an optical amplifier for operation with a required tilt or curvature within predetermined limits is provided, comprising the steps of:
providing an optical amplifier having length of rare-earth doped optical fibre and having an input port and an output port;
varying the temperature of the rare-earth doped optical fibre to a plurality of different temperatures;
determining at the different temperatures different measures of tilt or curvature present at the output port of the amplifier in the presence of an input signal at the input port; and, providing feedback related to the measure of tilt at at least one of the plurality of different temperatures.
In accordance with the invention, there is provided, a plurality of optical amplifiers having a substantially same tilt in the presence of a same input optical signal and having a substantially different temperature set point at which the erbium optical fibre is maintained.
In accordance with an aspect of the invention, an optical amplifier kit is provided, comprising:
a) an optical amplifier for providing a tilt varying with temperature having:
a length of rare-earth doped optical fibre;
a pump laser optically coupled thereto;
at least one of a heater and a cooler coupled to the rare-earth doped optical fibre for maintaining the optical fibre at a predetermined temperature corresponding to a predetermined tilt or curvature of the optical amplifier; and,
b) a measure of the temperature at which the amplifier is to be maintained to obtain an output tilt corresponding to the predetermined tilt.
In accordance with another aspect of the invention, a method of controlling an optical amplifier is provided for operation with a required tilt or curvature within predetermined limits, comprising the steps of: providing an optical amplifier having length of rare-earth doped optical fibre and having an input port and an output port; and providing a feedback signal related to the measure of tilt or curvature while varying the temperature of the rare-earth doped optical fibre.